The osteogenic commitment of CD271+CD56+ bone marrow stromal cells (BMSCs) in osteoarthritic femoral head bone.

Osteoarthritis (OA), the most common joint disorder, is characterised by progressive structural changes in both the cartilage and the underlying subchondral bone. In late disease stages, subchondral bone sclerosis has been linked to heightened osteogenic commitment of bone marrow stromal cells (BMSCs). This study utilised cell sorting and immunohistochemistry to identify a phenotypically-distinct, osteogenically-committed BMSC subset in human OA trabecular bone. Femoral head trabecular bone tissue digests were sorted into CD45-CD271+CD56+CD146-, CD45-CD271+CD56-CD146+ and CD45-CD271+CD56-CD146-(termed double-negative, DN) subsets, and CD45+CD271-hematopoietic-lineage cells served as control. Compared to the CD146+ subset, the CD56+ subset possessed a lower-level expression of adipocyte-associated genes and significantly over 100-fold higher-level expression of many osteoblast-related genes including osteopontin and osteocalcin, whilst the DN subset presented a transcriptionally 'intermediate' BMSC population. All subsets were tri-potential following culture-expansion and were present in control non-OA trabecular bone. However, while in non-OA bone CD56+ cells only localised on the bone surface, in OA bone they were additionally present in the areas of new bone formation rich in osteoblasts and newly-embedded osteocytes. In summary, this study reveals a distinct osteogenically-committed CD271+CD56+ BMSC subset and implicates it in subchondral bone sclerosis in hip OA. CD271+CD56+ subset may represent a future therapeutic target for OA and other bone-associated pathologies.

The simultaneous analysis of mesenchymal stem cells and early osteocytes accumulation in osteoarthritic femoral head sclerotic bone.

OBJECTIVE: OA subchondral bone is a key target for therapy development. Osteocytes, the most abundant bone cell, critically regulate bone formation and resorption. Their progenitors, mesenchymal stem cells (MSCs), display altered behaviour in osteoarthritic subchondral bone. This study investigated the relationships between native osteocytes and native MSCs in osteoarthritic femoral heads. METHODS: To avoid culture manipulations, a bone treatment procedure was developed to simultaneously obtain pure osteocyte-enriched fragments and matched native CD45-CD271+ MSCs. Gene expression in osteocytes and MSCs was compared between healthy and OA bone and selected molecules were examined by immunohistochemistry in relation to OA tissue pathology. Cell sorting and standard trilineage differentiation assays were employed to test OA MSC functionality. RESULTS: Native osteocyte enrichment was confirmed histologically and by higher-level osteocyte maturation transcripts expression, compared with purified MSCs. Compared with healthy bone, native OA osteocytes expressed 9- and 4-fold more early/embedding osteocyte molecules E11 and MMP14, and 6-fold more osteoprotegerin (P<0.01). CD271+ MSCs accumulated in the regions of bone sclerosis (9-fold, P<0.0001) in close juxtaposition to trabeculae densely populated with morphologically immature E11-positive osteocytes (medians of 76% vs 15% in non-sclerotic areas, P<0.0001), and osteoblasts. Gene expression of OA MSCs indicated their bone formation bias, with retained multipotentiality following culture-expansion. CONCLUSIONS: In human late-stage OA, osteogenically-committed MSCs and adjacent immature osteocytes exhibit a marked accumulation in sclerotic areas. This hitherto unappreciated MSC-early osteocyte axis could be key to understanding bone abnormalities in OA and represents a potential target for novel therapy development in early disease.

Bone Marrow-Harvesting Technique Influences Functional Heterogeneity of Mesenchymal Stem/Stromal Cells and Cartilage Regeneration.

BACKGROUND: Connective tissue progenitors (CTPs) from native bone marrow (BM) or their culture-expanded progeny, often referred to as mesenchymal stem/stromal cells, represents a promising strategy for treatment of cartilage injuries. But the cartilage regeneration capacity of these cells remains unpredictable because of cell heterogeneity. HYPOTHESIS: The harvest technique of BM may highly influence stem cell heterogeneity and, thus, cartilage formation because these cells have distinct spatial localization within BM from the same bone. STUDY DESIGN: Controlled laboratory study. METHODS: CTPs obtained from the femur of patients undergoing total hip replacement by 2 harvest techniques-BM aspiration and BM collection-after bone rasping were immunophenotyped by flow cytometry and evaluated for chondrogenic ability. The spatial localization of different CTP subsets in BM was verified by immunohistochemistry. RESULTS: Cells from the BM after rasping were significantly more chondrogenic than the donor-matched aspirate, whereas no notable difference in their osteogenic or adipogenic potential was observed. The authors then assessed whether distinct immunophenotypically defined CTP subsets were responsible for the different chondrogenic capacity. Cells directly isolated from BM after rasping contained a higher percentage (mean, 7.2-fold) of CD45-CD271+CD56+ CTPs as compared with BM aspirates. The presence of this subset in the harvested BM strongly correlated with chondrogenic ability, showing that CD271+CD56+ cells are enriched in chondroprogenitors. Furthermore, evaluation of these CTP subsets in BM revealed that CD271+CD56+ cells were localized in the bone-lining regions whereas CD271+CD56- cells were found in the perivascular regions. Since the iliac crest remains a frequent site of BM harvest for musculoskeletal regeneration, the authors also compared the spatial distribution of these subsets in trabeculae of femoral head and iliac crest and found CD271+CD56+ bone-lining cells in both tissues. CONCLUSION: Chondrogenically distinct CTP subsets have distinct spatial localization in BM; hence, the harvest technique of BM determines the efficiency of cartilage formation. CLINICAL RELEVANCE: The harvest technique of BM may be of major importance in determining the clinical success of BM mesenchymal stem/stromal cells in cartilage repair.

Targeting subchondral bone mesenchymal stem cell activities for intrinsic joint repair in osteoarthritis.

Osteoarthritis (OA) is a common age-related disease with complex pathophysiology. It is characterized by wide-ranging tissue damage and ultimate biomechanical failure of the whole joint. However, signs of tissue adaptation and attempted repair responses are evident in OA-affected osteochondral tissues. Highlighted in this review article is the role of bone-resident mesenchymal stem cells (MSCs) in these bone remodeling responses, and a proposal that targeting MSC activities in OA subchondral bone could represent a novel approach for intrinsic joint regeneration in OA. The development of these therapies will require better understanding of MSC proliferation, migration and differentiation patterns in relation to OA tissue damage and further clarification of the molecular signaling events in these MSCs during disease progression.